QST104 Datasheet, PDF (11/45 Pages) STMicroelectronics – Capacitive touch sensor device 4 keys with individual key state outputs or I2C interface

QST104
4
Device operating modes
Device operating modes
4.1
Reset and power-up
At power-up, the device configures itself according to the pull-up or pull-down option
resistors present on pins OPT1 to OPT6. The device start-up and configuration may take up
to t_Setup.
When the power is established, it is possible to force a new device configuration by applying
a negative pulse on the RESET pin.
The RESET pin is a bidirectional pin with an internal pull-up. The line is forced low when the
device resets itself (through an I²C command, for example).
A 10nF capacitor is recommended on the RESET pin to ensure reliable start-up and noise
immunity.
4.2
Burst operation
The device operates in “Burst” mode. Each key touch is acquired using a burst of charge-
transfer sensing pulses whose count varies depending on the value of the sense capacitor
C_S and the load capacitance C_X.
In Low Power mode, the device sleeps in an ultra-low current state between bursts to
conserve power.
4.3
Low power mode
In order to reduce the device power consumption, the QST family include scalable low
power modes.
● Standard low power mode
When the device is in standard low power mode, a window with very low power
consumption is inserted between the acquisition of the last active key and the following
acquisition of the first active key.
This window duration is programmable as the 'sleep duration time'.
Note that the sleep window insertion is cancelled in the following conditions:
– If a change is detected on a key, in order to speed up the DI process, the sleep
window insertion is skipped until the end of the DI process.
– In I²C mode, when a key change is actually detected and reported with a negative
pulse on the IRQ pin. In this case, the low power mode is disabled until a
command is received from the host.
– Inside an I²C command, between the Write and the Read I²C frames, the sleep
period is skipped.
● Free run in detect
The behavior in this mode is the same as in the standard low power mode except that
the sleep window insertion is always skipped if any of the active keys is detected as
touched.
This is useful to improve the wheel response time.
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